Method for determining the power of an intraocular lens

ABSTRACT

A method for calculating the power of an intraocular lens including measuring an axial separation between the front surface of the cornea and the plane of the iris root; and determining the power of the intraocular lens using the measured axial separation together with other measured parameters and empirically determined lens constants.

RELATED APPLICATION

This application is a division of application Ser. No. 14/835,024, filedAug. 25, 2015, entitled “Method for Determining the Power of anIntraocular Lens,” which in turn is a division of application Ser. No.12/630,656, filed Dec. 3, 2009, entitled “Method for Determining thePower of an Intraocular Lens”, now U.S. Pat. No. 9,144,375, issued Sep.29, 2015, which in turn is a continuation of PCT/EP2008/004406, filedJun. 3, 2008, which claims the benefit of U.S. Provisional ApplicationNo. 60/993,012, filed Jun. 4, 2007, each of which is hereby fullyincorporated herein by reference.

TECHNICAL FIELD

Biometry, particularly measurement of geometrical parameters in theanterior segment, for the calculation of the refractive power ofintraocular lenses (IOL).

BACKGROUND OF THE INVENTION

For the pre-operative calculation of the power of an intraocular lens,three input parameters are needed: the axial length of the eye (AL), therefractive power of the cornea, and the distance between the front ofthe cornea and the back focal plane of the intraocular lens, theso-called effective lens position (ELP).

To a good approximation, the post-operative axial length can besubstituted by the corresponding value measured pre-operatively. Theaxial length can be measured either ultrasonically or optically usingpartial coherence interferometry (PCI). Also at least for eyes that havenot undergone keratorefractive surgery the post-op corneal power can bepredicted based on the pre-op measurement of the front surface cornealradii. This prediction is based on assumptions about the corneal indexof refraction and the ratio of front and back surface corneal radii.Keratometry can be measured using manual or automatic opticalkeratometers, or extracted from a corneal topography obtained viaPlacido ring projection.

The effective lens position, on the other hand, is inherently apost-operative value. In fact the final position of an IOL does notmanifest itself until a number of weeks after surgery, when the capsularbag has shrunk around the implant. A pre-op parameter the ELPapproximately corresponds to is the distance from the front of thecornea to the front of the crystalline lens, the so-called anteriorchamber depth (ACD). The ACD can be measured ultrasonically or opticallyusing slit projection, or it can be predicted based on the diameter ofthe clear cornea (the so-called white-to-white distance, WTW) and itscentral curvature. In commonly used IOL calculation formulas, the ELP ispredicted using an empirical fit of several parameters such as ACD andAL. Olsen has suggested that the prediction can be improved by inclusionof additional parameters such as the lens thickness (LT), cornealradius, and pre-op refraction (Acta Ophthalmol. Scand. 2007: 85: 84-87).Most commonly used IOL calculation formulas are based on the samevergence formula to model focusing by the intraocular lens; they onlydiffer in the method for predicting ELP.

SUMMARY OF THE INVENTION

It is the purpose of this invention to provide a better ELP predictionby measuring a pre-op quantity that correlates closely to the post-opposition of the IOL. In one preferred embodiment, an OCT device is usedto identify the iris root. The axial separation (ACD′) between the frontsurface of the cornea and the plane of the iris root is then determined.The power of the intraocular lens is determined using the measured axialseparation together with other measured parameters and empiricallydetermined lens constants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of the eye.

FIG. 2 is a cross sectional view of the eye illustrating the axialdistance (ACD′) between the front surface of the cornea and the plane ofthe iris root.

DETAILED DESCRIPTION OF THE DRAWINGS

As seen in the attached FIG. 1, in cross-sectional images of theanterior segment of the eye obtained by optical coherence tomography(OCT) at infrared wavelengths (e.g., 1310 nm) a strongly scatteringlayer 10 is visible near the back surface of the iris. This structure iscommonly interpreted as the iris pigment epithelium. On the other hand,it has also been suggested that this scattering region may correspond tothe iris dilator muscle.

At the periphery, the absorbing layer ends at a well-defined radialposition (location 12 in FIG. 1), which is anatomically close to, orco-located with the iris root. In a meridional cross sectional OCT scanof the cornea the two peripheral end points of the scattering layer inthe iris can be used to uniquely identify two iris root points. A lineconnecting the two iris root points can be used to define a root-to-rootline. This line is shown as item 20 in FIG. 2.

The separation of the root-to-root line from the front of the cornea 16can be used to define a modified anterior chamber depth parameter ACD′(see FIG. 2). This can be defined in one of several different ways. Mostsimply, ACD′ can be defined as the longest perpendicular distance fromthe root-to-root line to the front surface of the cornea. If the patientfixates in a direction parallel to the direction of the OCT scan duringmeasurement, the line of sight can be uniquely identified in theacquired cross sectional image. ACD′ can then alternatively be definedas the longest distance from the root-to-root line to the corneal frontsurface, measured parallel to the line of sight. Finally, the innerlimits of the iris in the OCT scan can be used to mark the mid-point ofthe pupil along the root-to-root line. As a third alternative, theparameter ACD′ can be measured from this mid-point to the corneal frontsurface, in a direction parallel to the line of sight.

The parameter ACD′ can be used to predict the post-op effective lensposition. Like with presently used formulas this can be done byempirically determining regression coefficients for a set of parameters,such as ACD′, AL, and/or LT. Other measured parameters can include thetwo central radii of the corneal front surface. The prediction of ELPthus obtained can be used in IOL calculation formulas together withempirical lens constants.

Another possible approach relies on a measurement associated with aregion located at the most anterior portion of a highly scattering layerposterior to the sclera (location 14 of FIG. 1). This layer ispresumably a pigmented layer along the posterior boundary of the ciliarymuscle. Similar to the approach shown in FIG. 2, a line connecting thesetwo opposing points (14) can be drawn and the separation between thisline and the front of the cornea (ACD″—not shown) can be defined andused to predict the IOL position after surgery. The value for ACD″ canbe used alone or in conjunction with the value for ACD′.

As is well known in the art, the determination of regressioncoefficients requires large data sets and produce formulas that havelimited physical interpretation. The larger number of measurements to beincluded and the more complex the formula, the more data is required todevelop those formulas. This can especially be a drawback in themodification of IOL calculation formulas for newly developed IOL's. TheIOL calculation formula may instead take the form of regression formulasto calculate intermediate parameters such as the position of the IOLequator and the effective power of the lens. For example, the ELP isdetermined by a combination of anatomical features, such as the distancefrom the corneal vertex to the sulcus, by the design of the IOL and bysurgical technique. Various surrogate measurements may be combined. Forexample the ACD′ characterizes the position of the iris root. Acombination of the traditional ACD, LT, anterior radius of curvature ofthe crystalline lens, and possibly also posterior radius of curvature ofthe crystalline lens, characterize the crystalline lens equator. Theseand other surrogate measurements (including ACD″) can be combined into aregression formula for predicting the position of the IOL equator. TheELP prediction can then be calculated as a combination of the opticalpower, derived from the radii of curvature and index of refraction, thepredicted IOL equator. The resulting ELP estimate can be integrated intoan IOL calculation formula.

A particular embodiment of the inventive method consists in thefollowing sequence: The axial length (AL) of a patient eye is measuredusing partial coherence interferometry (PCI), the modified anteriorchamber depth ACD′ is determined using optical coherence tomography(OCT) and the corneal power is determined using a suitable keratometricsetup. The keratometric setup can be a stand-alone keratometer orintegrated into a combination device such as the IOLMaster. Afterobtaining these measurements, the values are processed together with thedesired target refraction using the Haigis-Formula to determine therequired power of an intraocular lens.

In the Haigis-Formula

${DL} = {{\frac{n}{L - d} - {\frac{n}{{n\text{/}z} - d}\mspace{14mu} {with}\mspace{14mu} z}} = {{{DC} + {\frac{ref}{1 - {{ref}\mspace{14mu} {dBC}}}\mspace{14mu} {and}\mspace{20mu} {DC}}} = \frac{{nC} - 1}{RC}}}$

where

DL: IOL-refraction

DC: cornea refraction

RC: cornea radius

nC: refractive index of the cornea

ref: refraction to be obtained after surgery

dBC: spectacle distance from cornea

d: optical anterior chamber depth ACD

L: Eye length

n: refractive index of the eye (1.336)

d is normally predicted using a function based on a multi-variableregression analysis from a large sample of surgeon and IOL-specificoutcomes for a wide range of axial lengths (AL) and anterior chamberdepths (ACD).

In the preferred embodiment, the modified anterior chamber depthsparameter ACD′ (or ACD″) would be used in place of ACD in the regressionfit.

In other common IOL formulas an expression equivalent to d is used tooas the table shows:

SRK/T d = A-constant Hoffer Q d = pACD Holladay 1 d = Surgeon FactorHolladay 2 d = ACD

Also in these formulas d may be substituted by the modified anteriorchamber depth ACD′ (or ACD″).

The invention is not limited to the embodiments described, also otheruses of the measured values ACD′ or ACD″ for IOL calculation fall withinthe scope of protection.

1. A method for calculating a power of an intraocular lens comprisingthe steps of measuring an axial separation (ACD′) between a frontsurface of a cornea and a plane of an iris root; and determining thepower of the intraocular lens using the measured axial separationtogether with other measured parameters and empirically determined lensconstants.
 2. The method of claim 1, wherein the plane of the iris rootis determined using an optical coherence measurement at an infraredwavelength.
 3. The method of claim 2, wherein the optical coherencemeasurement comprises one or more meridional scans.
 4. The method ofclaim 3, wherein the location of the iris root is measured as outer endpoints of a strongly scattering layer in back of the iris.
 5. The methodclaim 4, wherein the other measured parameters include an axial eyelength and at least two central radii of the corneal front surface. 6.The method of claim 5, where the other measured parameters furtherinclude a lens thickness.
 7. A method for calculating a power of anintraocular lens comprising steps of: measuring an axial separation(ACD″) between a front surface of a cornea and a line connectingposterior edges of a ciliary body; and determining the power of theintraocular lens using the measured axial separation together with othermeasured parameters and empirically determined lens constants.
 8. Themethod of claim 7, wherein the line connecting a posterior edge of aciliary body is determined using an optical coherence measurement at aninfrared wavelength.
 9. The method of claim 8, wherein the opticalcoherence measurement comprises one or more meridional scans.
 10. Themethod claim 7, wherein the other measured parameters include an axialeye length and at least two central radii of the corneal front surface.11. The method of claim 7, where the other measured parameters furtherinclude a lens thickness.